Flush-mounted balanced log-periodic antenna



July 5, 1966 P. F. STANG 3,259,905

FLUSH-MOUNTED BALANCED LOG-PERIODIC ANTENNA Filed April 15, 1964 4 Sheets-Sheet 1 INVENTOR. PAU L F. S TAN 6 July 5, 1966 STANG 3,259,905

FLUSH-MOUNTED BALANCED LOG-PERIODIC ANTENNA Filed April 15, 1964 4 Sheets-Sheet 2 FIG- 2 INVENTOR. PAUL F. STANG Agent July 5, 1966 P. F. STANG 3,259,905

FLUSH-MOUNTED BALANCED LOG-PERIODIC ANTENNA Filed April 15, 1964 4 Sheets-Sheet 4 PAUL F. STANG United States Patent 3,259,905 FLUSH-MOUNT ED BALANCED LOG-PERIODIC ANTENNA Paul F. Stang, Arleta, Califl, assignor ,to Lockheed Aircraft Corporation, Burbank, Calif. Filed Apr. 15, 1964, Ser. No. 359,857 5 Claims. (Cl. 343-7925) This invention relates to logarithmically periodic antennas and more particularly-to a balanced, flush-mount- .ed, logarithmically periodic antenna having a cavity backed tooth structurerwhich radiates in a quasi-end fire mode with a forward pointing beam and which is particularly suited for use in aerospace vehicles.

Antennas of the type known as log periodic antennas are well-known in -the art and generally have two parts which are fed against each other, both halves of which radiate when ptheantenna is mounted in :free space.

There are, however, many applications in which an antenna system must .mount flush in a surface, radiating 'into only the half-space on one side {the fuselage of an airplane, for example). In these cases, a terminating cavity must beincorporated on the side from which radiation is not desired. In order to have radiation from only one-ha'lf-of the structure, a terminating cavity must be provided .on the side from which radiation is to be suppressed. Attempts have been made heretofore to replace .one-half of the structure with such a cavity. How- .ever, over the desired frequency band of :1 the results have been unsatisfactory because of cavity resonance, and therefore the radiation patterns are found to be multilobed at certain frequencies. This means that the antenna cannot be halved and have the same performance as a balanced model.

I-Ieretofore, reactive terminating cavities have been used successfully over hands up to approximately 2:1. However, prior to the present invention a terminating cavity suitable for antennas operating over a 10 to 1 band has not beenachieved. The cavity to be completely effective must absorb one-half of the radiated power at all frequencies of operation. Under this condition, the performance of the antenna wvould be the same as its free space performance in the hemisphere of radiation. The directivity is doubled while the etiiciency is halved and the gain, in a single hemisphere, is unchanged. 'In the present invention a novel resistive cavity termination rather than a reactive "termination is employed.

According to the present invention, there is provided an antenna which avoids resonance of the cavity and in which a balanced type of antenna is used. Both ,antenna halves are inserted, with one of the antenna halves being located inside the non-reflecting cavity and the other being flushmounted in the cavity lid. The feed point is at the vertex of the two antenna halves and is not connected with the cavity? The cavity is lined on all sides with a non-reflecting material to provide a resistive, rather than a reactive, terminating cavity.

It is important to note, however, that both antenna halves radiate the energy. The flush-mounted antenna halfradiates the energy into free space, and the one inside of the cavity radiates its energy into the non-reflecting material. This portion of the antenna looks like a matched dummy antenna. The whole antenna system is therefore wellba'lanced, and no resonance from the cavity occurs.

Log periodic antenna systems may -be described generally as consisting of individual antenna elements each of which is generally triangular in shape having a vertex, and having side elements defined by an angle on extendingffrom the vertex. Eachantenna element is comprised of at least two radial sections defined on one side by the centerline of the antenna element and on the other side by a radial line extending from the vertex at an angle a/Z with respect to the centerline of the element. Each radial sect-ion has a plurality of teeth comprised of elements generally transverse to the centerline of the antenna element. These teeth are all similar to one another in shape, but become progressively larger and spaced progressively farther apart as the distance from the vertex increases. The relationship of the elements may be expressed by stating that the radial distance from the vertex to any given tooth in a given radial section bears a constant ratio 1- to the radial distance of a corresponding point on the next adjacent tooth which is \farther removed from the vertex than said given tooth. In a conventional embodiment, the antenna elements are employed in multiples of two with a pair of such elements being positioned with respect to each other so that the vertices are positioned near each other (but electrically separated) and extending out from the common vertex in such a manner as to define opposite sides of a pyramidal-shaped structure. The angle between the opposing antenna elements is given as p. The beamwidth may be controlled to a considerable extent by a geometry of the structure.

An object of the present invention is to provide a novel and improved log periodic antenna which may be flushmounted as in a fuselage of an aerospace vehicle.

A further object .of the invention is to provide a novel and improved balanced log periodic antenna employing a resistive cavity.

Still another object of the invention is to provide a balanced, flush-mounted, log periodic antenna having a cavity-backed tooth structure radiating in a quasi-end fire mode.

Another object .of .the invention is to provide a novel log periodic antenna having a self-contained matching transformer.

It is yet another object of the invention to provide a log periodicantenna having a novel and improved taperedline balun for matching the antenna to a low impedance transmission cable.

A general object of the invention isto provide a novel and improved log periodic antenna which overcomes disadvantages of previous means and methods heretofore intended ,to accomplish generally similar purposes.

The invention will be understood more completely .from the following detailed description, taken in conjunction with the drawings, in which:

FIGURE 1 .is aperspective viewof a flush-mounted balanced log periodic antenna constructed in accordance with the present invention;

FIGURE 2 shows a single antenna element according to the invention having a plurality of .trapezoidally shaped transverse dipole elements extending outward from a vertex;

FIGURE 3 shows two antenna elements .of the type shown in FIGURE 2 arranged in a non-,planararray with a common vertex;

FIGURE 4 shows a cavity having anon-reflecting .interior covering for receiving the elements shown in FIG- URE 3;

FIGURE 5 is a plan view of the coaxiallogarithmictapered line transformer;

FIGURE 6 is an elevation view of the apparatus of FIGURE 5; and

FIGURE-7 is a graphic plot of the VSWR of the antenna of-F'IGURE 1.

As stated hereinabove, the present invention relates to an antenna structure in which the input impedance and radiation patterns vary periodically with the logarithm of "the frequency. For a particular class of such structures which in the exemplary embodiment is 15.

the variation of the electrical characteristics over a period is negligible, the result being an antenna for which the impedance and patterns are essentially independent of frequency over bandwidths greater than 1. Looking now at FIGURE 2 there is shown a conductor element 1 having a trapezoidal tooth structure comprising one-half of the antenna. The pattern shown may be cut from a conduc bodiment shown in FIGURE 2 has eleven teeth. Other dimensions of the exemplary embodiment are as follows:

The width of each tooth is defined by the radial distance between R and r,,, where R is the tooth boundary farthest from the apex and r is the tooth boundary closest to the apex 3. The low-frequency cutolf occurs, approximately, when the longest tooth is A wavelength long and the highfrequency cutoff occurs when the shortest tooth is somewhat less than Mr wavelength long. Two conductors of the type shown in FIGURE 2 are arranged as shown in FIGURE 3 to provide a balanced array. The apex of each half of the antenna is provided with feed terminals 5 and 6, respectively.

In a typical practical construction, the angle 1 between the two halves of the antenna is approximately The impedance of such an arrangement shifts with the vertex angle 1// and the angle a of the structure. Referring again to FIGURE 2, it can be seen that the conductor element 1 is generally triangular in shape with the base dimension of the triangle in the exemplary case being 5.85 inches. The notches which divide the two sides of the triangle into a plurality of teeth are trapezoidal in shape, as are the teeth themselves. The centerline of each trapezoidal section lies along thesame line as one of the radially extending edges of its adjacent trapezoidal sections. Thus, the trapezoidal sections are alternately displaced from the centerline of the triangle.

Log periodic antennas generally have elements identical to their complements and, as a result, have input impedances of approximately 180 ohms. For the best match, the line feeding the antenna must have a characteristic impedance near this value. However, it would be desirable if the input impedance of the periodic structure could be varied to match the characteristic impedance of conventional, commercially available transmission lines. In a preferred construction the antenna conductors 1 and 4 of FIGURE 3 are matched to a 50 ohm cable by means of a novel transformer comprising a Teflon-filled logarithmically tapered line balun. V V V The balun converts from a coaxial line (unbalanced line). to a two-wire transmission line (balanced line). The Teflon filling increases the electrical length by the dielectric constant x/e. The logarithmic taper will match the antenna more perfectly to a commonly used 50 ohm line than will a linear taper. The balun is'shown in FIG- URE 4. The first feature of the Teflon-filled line is that the outer conductor 7 is logarithmically opened .up. Second, the electric length of the-structure is increased by the dielectric constant. Both of these features improve the impedance match. Furthermore, the mechanical 4 strength of the apparatus is improved, making it more resistant to vibration as usually encountered in aerospace applications. In a typical construction, the balun comprises a coaxial cable having its outer shield conductor 7,

tapered down over a suitable distance, which in a practical construction is approximately one foot, at which end, it is terminated. The inner conductor 8 is enclosed within a Teflon dielectric member 9 of generally cylindrical shape. A suitable connector 11 is attached to the line; end of the balun. Details of construction can also be seen in FIGURES 5 and 6. The low impedance terminus of conductor 7 is shown at 7a in FIGURES 5 and 6, while the high impedance terminus is indicated at 7b. Similarly,

the low impedance terminus of inner conductor 8 is shown at St: in FIGURES 5 and 6, and the high impedance terminus is indicated at 8b. The Teflon dielectric is indicate at 9 in FIGURE 6.

The two antenna halves (elements 1 and 4) are fed against each via the tapered line balun. The upper antenna element 1 is supported by a dielectric sheet '10 which encloses the top of the antenna cavity enclosure (see FIGURE 1). The antenna structure may be mounted, by any suitable means, so that the exterior surface of sheet 10 and antenna element 1, carried thereon, will be flush with the surroundingsurface of the supporting struc-, This surrounding surface may, for example, be I the skin of an aircraft, missile or satellite. The balun ture.

runs along the center of the antenna halves. The outside conductor 7 is connected to antenna element 4 which is located inside of the cavity. The inner conductor 8 is connected at the upper antenna element 1. FIGURE 4 shows the absorbing cavity enclosure which comprises a pair of sidewalls 12 and 13 adjoining endwall 14.1 The base of the cavity enclosure is bounded by a short'base section 15 which is parallel to the upper surface of the structure, and by an upwardly extending section 16. Connector 11, to which the balun 16 is attached, is supported by endwall 14. Conductors 7 and 8 extend from connector 11 and connect to the vertices 5 and 6 of antenna elements 1 and 4,. respectively. The cavity enclosure may be fabricated from any suitable material, either metallic or nonmetallic.

To minimize cavity resonance, the inside of the cavity is covered with laminated sheets 17-20 of absorbing material. Suitable material is commercially avaiable from Emerson &.Cumrnings, Inc., Canton, Mass. and is identified as type AN73 Two layers of this material are applied, but the contacting sheet is scraped oflflat the first layer and a coated cardboard inserted between the two layers of the absorbing material. This ismade to increase the bandwidth of the absorbing material. By this means, the cavity resonance is practically eliminated over a wide frequency band. At the lower end of the frequency band cavity resonance is not critical. At'the higher end of the frequency band .the cavity resonance; without the non-reflecting sheet would otherwise. be The size of the exemplary embodiment is 11.3 inches long, 8.0 inches wide and 3.0 inches deep. This antenna covers the frequency range from 1.0 gc. to i 10 gc. The voltage standing wave ratio (VSWR) is less 1 than 3:1 over the band. FIGURE 4 shows a VSWR- plotted as a function of frequency in stepsof500 mega:

troublesome.

cycles. As can be seen, the maximum VSWR is 3:1 at approximately 8 gc. quency band from 1 gc. to 10 gc.

angle of the structure in the same manner as in wellknown log periodic antennas.

An antenna in accordance. withthe invention is par-- ticularly suited for application in electronic counter: measures.

This pilot extends over the 1f-re-" The lobe of the radi: ation pattern is nearly the same as all of the test fre-A Also, the beam can be broadened or narrowed, and the takeoff angle can be varied with the vertex angle between the two antenna halves.

While there has been shown and described and pointed out the fundamental novel features of the invention as applied to a preferred embodiment, it will be understood that various omissions and substitutions and changes in the form and details of the device illustrated and in its operation may be made by those skilled in the art, without departing from the spirit of the invention; therefore, it is intended that the invention be limited only as indicated by the scope of the following claims.

What is claimed is:

1. In an antenna array:

(a) a pair of substantially triangularly shaped log periodic structures passing through the apex of the triangle and the midsection of the base, and antenna elements each having a centerline each comprising a plurality of radial sections having a generally trapezoidal shape;

(b) the centerline of each trapezoidal section being radially aligned with one edge of the adjacent trapezoidal section, the two antenna elements being angularly spaced apart so as to have their apexes extending towards a common vertex;

(c) a resistive cavity enclosing one of said antenna elements and having an open side which is co-planar with the other of said antenna elements; and

(d) transmission line means connected to each of said antenna elements at said apexes.

2. An antenna array as defined in claim 1 in which said trapezoidel sections are formed of a conductive sheet, the radial distance from said vertex of the antenna to any given point on any given trapezoidal section hearing a constant ratio to the radial distance from said vertex to the corresponding point on the next adjacent trapezoidal section farther removed from said vertex and on said given radial section.

3. An antenna array as defined in claim 1 in which said transmission line means comprises:

(a) a central conductor;

(b) a solid cylindrical dielectric coaxially enclosing said central conductor;

(c) an outer conductor coaxially enclosing one end of said central conductor and being logarithmically opened in its length in a direction towards the other end of said central conductor; and

(d) the opened end of said outer conductor being connected to said one antenna element and said other end of said central conductor being connected to said other antenna element.

4. An antenna apparatus comprising:

(a) first and second antenna elements angularly spaced apart and having a common vertex, each of said elements being generally triangular in shape slots alternately positioned on opposite sides of the centerline of said element and having a fixed ratio of edge to centerline dimensions;

(b) a resistive cavity enclosing one of said elements and having one exterior surface which is coplanar with the other of said elements; and

(c) tranmission line means connected to said element at said vertex.

5. An antenna apparatus as defined in claim 4 wherein said cavity comprises:

(a) an enclosure having a plurality of flat side enclosing walls;

(b) electromagnetic energy absorbing means secured to all but one of said walls; and

(c) said other of said elements being secured to said one wall.

References Cited by the Examiner UNITED STATES PATENTS 3,192,531 6/1965 Cox et a1. 343-895 ELI LIEBERMAN, Acting Primary Examiner.

R. F. HUNT, Assistant Examiner. 

1. IN AN ANTENNA ARRAY: (A) A PAIR OF SUBSTANTIALLY TRIANGULARLY SHAPED LOG PERIODIC STRUCTURES PASSING THROUGH THE APEX OF THE TRIANGLE AND THE MIDSECTION OF THE BASE, AND ANTENNA ELEMENTS EACH HAVING A CENTERLINE EACH COMPRISING A PLURALITY OF RADIAL SECTIONS HAVING A GENERALLY TRAPEZOIDAL SHAPE; (B) THE CENTERLINE OF EACH TRAPEZOIDAL SECTION BEING RADIALLY ALIGNED WITH ONE EDGE OF THE ADJACENT TRAPEZOIDAL SECTION, THE TWO ANTENNA ELEMENTS BEING ANGULARLY SHAPED APART SO AS TO HAVE THEIR APEXES EXTENDING TOWARDS A COMMON VERTEX; (C) A RESISTIVE CAVITY ENCLOSING ONE OF SAID ANTENNA ELEMENTS AND HAVING AN OPEN SIDE WHICH IS CO-PLANAR WITH THE OTHER OF SAID ANTENNA ELEMENTS; AND (D) TRANSMISSION LINE MEANS CONNECTED TO EACH OF SAID ANTENNA ELEMENTS AT SAID APEXES. 